Objects and Arrays
GEEN163
“It's not at all important to get it right the
first time. It's vitally important to get it
right the last time.”
-Andrew Hunt and David Thomas
Course Evaluations
• Course evaluations are available on Blackboard
• Be sure to complete all evaluations for all classes
• 22% of the student in GEEN163 have completed
the course evaluation
Website Updated
• The GEEN163 website,
http://williams.comp.ncat.edu/geen163/
has been updated
Programming Project
• This week's program should be done by teams of two
students
• There are three classes, each with several small
methods
• The program involves a GUI with graphics
You CAN do it
Arrays of Objects
• The elements of an array can be primitive type like int,
double, long, float, char or boolean
• An array can also have objects as elements
Widget[] things = new Widget[47];
• This creates an array, things, that can contain 47
objects of the class Widget
Empty Array
• When you create an array of double or int, the array
initially contains many doubles or ints
• When you create an array of objects, the array is
initially empty.
Creating Objects from Files
• A program might declare an array of objects in the
beginning and then create the object later
• Consider a program that reads a data file and creates
an object for each line of data
What is Displayed?
String[] goat = new String[3];
String target = "";
for (int i = 0; i < 3; i++) {
if (goat[0].equals(target)) {
System.out.print( i );
}
}
A. 0 1 2
B. 0
C. 2
D. nothing
E. none of the above
Now What is Displayed?
String[] goat = new String[3];
String target = "";
for (int i = 0; i < 3; i++) {
if (target.equals(goat[0])) {
System.out.print( i );
}
}
A. 0 1 2
B. 0
C. 2
D. nothing
E. none of the above
Example Class
• Consider a class that holds data
about classrooms
public class Classroom {
private String building;
private int
roomNum;
private int
capacity;
}
Constructor
• Here is a possible constructor for Classroom that
initializes all class instance variables to parameters
public Classroom( String name, int num, int cap) {
building = name;
roomNum = num;
capacity = cap;
}
Setters
• Write three short methods that set the value of the
three class instance variables
Possible Solution
public void setBuilding( String name ) {
building = name;
}
public void setRoomNum( int num ) {
roomNum = num;
}
public void setCapacity( int cap ) {
capacity = cap;
}
Write a Method
• Write a method that has a room name and number as
parameters and returns true when if this object
matches the name and number
Method Header, Write the Rest
• Write a method that has a room name and number as
parameters and returns true when if this object matches
the name and number
public boolean isRoom( String bldg, int num )
Possible Solution
public boolean isRoom(String bldg, int num ){
if (bldg.equalsIgnoreCase(building)
&& num == roomNum) {
return true;
}
return false;
}
Another Possible Solution
public boolean isRoom(String bldg, int num ){
return bldg.equalsIgnoreCase(building)
&& num == roomNum;
}
Building an Array of Objects
java.io.File frog = new java.io.File("mydata.txt");
java.util.Scanner inFile = new java.util.Scanner(frog);
Classroom[] space = new Classroom[1500]; // list of rooms
int numRooms = 0;
// number of rooms
while (inFile.hasNext()) {
String bldg = inFile.next();
int room = inFile.nextInt();
int cap
= inFile.nextInt();
space[numRooms] = new Classroom( bldg, room, cap);
numRooms++;
}
Search Example
• Assume we have an array of Classroom objects called
space with numRooms values in the array
• We want to display the capacity of a given room
• The input will be a building name and room number
Search Program
String name = keyboard.next();
int num = keyboard.nextInt();
look: for (int i = 0; i < numRooms; i++) {
if (space[i].isRoom( name, num )) {
System.out.println(space[i].getCapacity());
break look;
}
}
Search Program with extended for
String name = keyboard.next();
int num = keyboard.nextInt();
look: for (Classroom room : space) {
if (room != null && room.isRoom( name, num )) {
System.out.println(room.getCapacity());
break look;
}
}
What is displayed?
Classroom here;
space[5].setCapacity( 47 );
here = space[5];
here.setCapacity( 4 );
System.out.println( space[5].getCapacity() );
A. 4
B. 5
C. 47
D. None of the above
String Methods
• There are many methods that can be called on objects
of the String class
• length() Returns the length of this string
String lecture = “GEEN163”;
int howLong = lecture.length();
// 7
Searching Strings
• You can search a string to see if it contains the
another string
• indexOf( String target ) Returns the index within this
string of the first occurrence of the specified String or
-1 if not found
indexOf Example
• The position of a character in a string starts
counting at zero
String major = "Computer Science";
// 0123456789012345
int where = major.indexOf( "put" );
where = major.indexOf( "fail" );
// 3
// -1
Taking Strings Apart
• substring(int beginIndex, int endIndex) Returns a new
string that is a substring of this string. The substring
begins at beginIndex and extends to the character at
position endIndex - 1. Thus the length of the substring
is endIndex – beginIndex
• charAt(int index) Returns the char value at the
specified index
substring Example
String major = "Computer Engineering";
//
1111111111
// 01234567890123456789
String result = major. substring( 11, 14 );
result has the value "gin"
Changing Case
• toUpperCase() returns a copy of the String with all
of the letters in upper case
• toLowerCase() returns a copy of the String with all
of the letters in lower case
String gnu = "Today is 8/29/12";
String wildebeest = gnu.toUpperCase();
System.out.println(gnu +"\n"+wildebeest);
Today is 8/29/12
TODAY IS 8/29/12
What is the value of result?
String major = "Computer Engineering";
//
01234567890123456789
String result = "S" + major. substring( 12, 14 );
A. Sine
B. ine
C. Sin
D. in
E. S
Miscellaneous String Method
• trim() Returns a copy of the string, with leading and
trailing whitespace omitted
String hair = " What ever
";
// 01234567890123
String bald = hair.trim();
// bald is "What ever";
Interesting String Methods
• length
• indexOf
• substring
• charAt
• toUpperCase
• toLowerCase
• trim
Write with your Team
• Given a string containing a first name and last name,
break it into two strings, one with the first name and
the other with the last name
String full = "Barack Obama", first, last;
The result would be
first is "Barack" and last is "Obama"
Possible Solution
String full = "Barack Obama", first, last;
int blankPos = full.indexOf( " " );
first = full.substring( 0, blankPos );
last = full.substring( blankPos+1, full.length() );
Who Dunit?
• Many GUIs have more than one active object
• You can determine where the action was taken by
using the getSource() method of the event
• The result of getSource() should be compared to
each GUI object
public void actionPerformed(
java.awt.event.ActionEvent frog ) {
if ( frog.getSource() == GUIobject ) {
Example Use of getSource()
JButton doit = new JButton(“OK”);
JButton skip = new JButton(“cancel”);
---------------------------public void actionPerformed(
java.awt.event.ActionEvent toad) {
if (toad.getSource() == doit ) {
// do it
} else if (toad.getSource() == skip ) {
// don’t do it
Course Evaluations
• Course evaluations are available on Blackboard
• Be sure to complete all evaluations for all classes
• 22% of the student in GEEN163 have completed
the course evaluation
Website Updated
The GEEN163 website
http://williams.comp.ncat.edu/geen163/
has been updated
Programming Project
• This week's program should be done by teams of two
students
• There are three classes, each with several small
methods
• The program involves a GUI with graphics
• Today's lecture gave you all sorts of hints